Neymar scored two penalties but the visitors collapsed late on at the Stade du Moustoir, as Nigerian Terem Moffi netted an injury-time winner.
The loss saw PSG stay one point behind league leaders Lyon, although third-placed Lille could go two points clear when they host Dijon later on Sunday.
“We have to keep the positive aspects and move forward,” said former Tottenham manager Pochettino.
“Accidents happen in football. We have to keep working. It makes us realise that it can happen if we don’t play at 100 percent.”
It was the first defeat in six games as PSG coach for Pochettino since he took over from the sacked Thomas Tuchel.
The win takes struggling Lorient into the relegation play-off spot.
“(Moffi’s goal) was magical. Against PSG, it’s even crazier,” said Lorient’s Laurent Abergel, who scored the opening goal.
“For a Marseille fan like me, the first dream is perhaps to play for OM and the second to beat PSG.”
Lorient started brightly and had the first real shot in anger in the 30th minute, as Adrian Grbic’s long-range strike had to be pushed away by PSG goalkeeper Sergio Rico.
The hosts did take the lead six minutes later, though, as PSG failed several times to clear and Abergel picked out the top corner with the outside of his left boot.
Neymar levelled on the stroke of half-time from the penalty spot with his fifth league goal of the season.
PSG improved after the break and took the lead in the 57th minute as Houboulang Mendes, who also conceded the first penalty, brought down Mauro Icardi and Neymar again made no mistake.
It was the first time since Zlatan Ibrahimovic in 2015 that a PSG player had netted two spot-kicks in a Ligue 1 match.
The visitors pressed for a third, but Lorient goalkeeper Matthieu Dreyer made an excellent stop to keep out an Icardi header.
That proved to be crucial, as the stubborn home side equalised in the 80th minute.
Substitute Yoane Wissa scored an excellent individual goal, his sixth of his first full campaign as a top-flight player.
PSG went in search of a winner, but the closest they came saw full-back Layvin Kurzawa’s drive fly narrowly wide.
They over-committed in the second minute of added time, leaving no defenders within 60 yards of their own goal as Moffi was sent racing clean through.
The 21-year-old, on as a sub, slotted the ball into the bottom corner to spark wild celebrations for the home team.